This invention relates generally to electronic countermeasure systems and more particularly to systems of such type which use digital computers for determining the number of pulses received from each one of a plurality of jamming sources.
As is known in the art, early electronic counter measure (ECM) systems received signals from various RF energy jamming sources, sometimes referred to as "emitters", and presented the location of such emitters on a suitable display, such as a cathode ray tube (CRT). An operator would observe the CRT display, tune his receiver to separate in frequency each of the emitters, perform manual direction finding, select the appropriate emitter to be jammed and transmit an appropriate jamming modulation. Other early ECM systems used repeater jammers which transmitted all signals received in a fixed frequency passband. A short coming of these early ECM systems was that they did not provide an automatic emitter identification and prioritization.
In more modern ECM systems, the signal processing emitter identification and jamming assignment must generally be done automatically because the emitter environment, both friendly and hostile, may be so dense that a single operator would not generally be able to perform the emitter identification task. Such a modern ECM system generally includes receiving equipment, signal processing equipment, digital computer, displays, a signal generator, and transmission equipment. The receiving equipment and signal processing equipment convert various characteristics of each emitter into a digital word. The characteristics are typically the emitter's direction and frequency. These digital words are fed to the digital computer which provides appropriate display for the operator and also automatically assigns jamming resources on a somewhat optimum basis to the threatening emitters.
In order to establish whether received radio frequency signals are from a valid emitter or from noise or in order to calculate the pulse repetition interval (PRI) of such emitter it is sometimes necessary to determine the number of times pulses from each one of the emitters are received in predetermined intervals of time. As mentioned above, in modern ECM systems, as each emitter signal is received it is generally converted into a digital word, one portion of the digital word may represent the direction of the emitter and the other portion of the digital word may represent the carrier frequency of the RF energy produced by the emitter. Hence each digital word may be considered as providing an identification of the emitter since each different emitter may be considered as having a unique carrier frequency and direction. For example if a first digital word is produced in response to a first received pulse indicating that the emitter's RF energy has a direction of 150 degrees and a frequency of 3.79 GHZ and a pulse is subsequently received from an emitter having the same direction and the same frequency it is very likely that both pulses came from the same emitter. Hence it follows that by counting the number of times the same unique digital word is produced in a predetermined period of time the pulse repetition interval of the pulses produced by the same emitter may be determined. Unfortunately, in many environments the number of emitters and pulse repetition frequency of each emitter are so great that the rate of pulses received by the apparatus may be greater than the processing rate of the digital computer and hence such digital computer might not be able to keep up with each one of the received pulses with the result that information may be lost by the digital computer.